Over the years the controlled movement of aircraft light control surfaces such as the flaps on the aircraft wings have progressed from pure mechanical systems that linked the flight control surfaces with a pilot control lever in the cockpit to hydraulic servo motor arrangements controlled from the cockpit and these hydraulic servo motor arrangements have become increasingly complex. There is little doubt that the emerging sophistication in the hydraulic valve arrangements utilized to control the servo motor or motors involved have improved performance. However, the improved performance has carried with it the burden of increased weight and multiply dependent interconnected components that inherently fall prey to malfunction and lost motion between components. Simply stated, with more components in a system there is statistically a greater probability of malfunction and backlash due to lost motion. It has always been a basic desire of flight surface control systems to provide a precision movement of the flap or a control surface in response to the movement of a pilot controlled stick or lever in the aircraft's cockpit.
Typical of the earlier efforts is the aircraft control surface assembly and actuating mechanism of M. Brunner described in U.S. Pat. No. 2,820,600. The Brunner assembly is a relatively uncomplicated hydraulic-mechanical mechanism that includes a rotary hydraulic motor which assumes and is maintained in a stalled atitude during such times as certain operating parts of the mechanism assume predetermined relative positions. The Brunner arrangement requires that the hydraulic motor that drives the flap between a retracted and fully extended position be driven into a stalled condition after a fixed number of revolutions. The Brunner arrangement is unable, as the invention to be described hereinafter, to position the flight control surface at an intermediate position.
Some years later, G. E. Lichtfuss created an actuator system shown and described in U.S. Pat. No. 3,662,550. The Lichtfuss actuator system for flight control surfaces includes a drive shaft 20 for actuating the flight control surfaces, a pair of hydraulic motors 39, 40 for driving the shaft, separate control circuits, FIG. 2, for the motors 39, 40 supplied from separate sources of pressure fluid. A normally operable selector valve 52 allows for the simultaneous energization of both control systems to operate both motors to drive the shaft. A free wheeling valve 60, 62 in each circuit enables either motor to be driven by the shaft in event of pressure failure in its circuit. A feedback mechanism 46 is adapted to neutralize both hydraulic systems when appropriate flight control surface adjustment has occurred. The feedback arrangement 46 is coupled to a valve stem 140 in selector valve 52 which is controlled by a rotatable shaft 175. The shaft 175 is mounted for pivoted movement back and forth and has an arm 179 engaging valve stem 140. The shaft 175 is adapted to be pivoted through the medium of an arm 176 fixed on the shaft and recessed at 177 to receive a driving pin 178 projecting axially from a rotatable disc 180, having a partial internal ring gear 181 integral therewith. The ring gear 181 is part of a differential mechanism including a planet gear 183 on a rotatable carrier 184, adapted to be pivoted by a lever 186 subject to control by the aircraft pilot. The differential mechanism further includes a sun gear 190 rotatable with a worm gear 192. The worm gear is driven by a worm 193 on a shaft 194 having a gear 195 adapted to be driven by a gear 196 on drive shaft 20. The Lichtfuss actuator system achieves position and control of the flight surfaces through the use of a complicated valving and a feedback mechanism described next above. The Lichtfuss system is further complicated by the incorporation of dual torque summing hydraulic systems. The invention to be described hereinafter provides all of the functional advances of the Lichtfuss actuator system, but with significantly fewer parts.